On another level, it’s evidence of a far more fascinating phenomena: As so-called personal fitness trackers like Jawbone’s UP become more ubiquitous, they can be used to track public events on an unprecedented scale.

Jawbone’s analysis showed, for instance, that 93 percent of users in Napa, Sonoma, Vallejo and Fairfield woke up suddenly at 3:20 a.m., the exact time the earthquake hit. About half of those people — 45 percent — stayed up the rest of the night. That second figure is particularly striking, as it essentially quantifies a heretofore unquantifiable feeling: collective anxiety. We can describe mass anxiety anecdotally; we have always been able to invoke the quotes of affected individuals, or ambiguous feelings from sentiment polls.

But the ability to go into thousands and thousands of people’s bedrooms, and know with certainly that after the earthquake, only half of them went back to sleep — that’s an incredibly intimate detail, an otherwise invisible pattern in disparate streams of personal facts.

More incredible: We increasingly have access to lots of insights of that kind. A number of presumed fitness-tracking companies are turning their (personal) data and insights to (very public) use: Jawbone, arguably the leader in this field, has churned out fascinating graphs on everything from what kind of foods Americans eat for breakfast to how late people stay up for the World Cup. Meanwhile, Fitbit has begun peddling its devices to large companies like BP, promising aggregate “group reports” on things like how far employees walk and how much they exercise.

When people in D.C. walk and sleep, per Jawbone UP data. (Jawbone)

That type of data, needless to say, has big applications in public health, as well: In May 2013, the California Institute for Telecommunications and Information Technology launched a “Health Data Exploration project,” which is — among other things — investigating how aggregate data from devices like Fitbit and Up could be used in health research.

“These devices are collecting ever-larger streams of data that have never been collected before on individuals,” researchers explain. For the first time in history, we can see things like a child’s real-time body temperature. Or a 70-year-old’s exercise habits. Or the exact impact of a surprise earthquake on the unsuspecting populace.

The only challenge now, they argue, is figuring out how to harness that data — and then to deploy it for social change.

What foods are typically eaten together, per Jawbone data. (Jawbone)

That said, personal data has always been a double-edged sword — and fitness tracking is no exception. Earlier this month, Sen. Chuck Schumer of New York (D) called fitness-tracking devices a “privacy nightmare” and asked the Federal Trade Commission to pursue action against companies that sell user data to advertisers and other third parties. (For what it’s worth, Fitbit — the company Schumer singled out — has said it’s never sold user data.) Meanwhile, Sen. Al Franken (D-Minn.) is leading a charge against smartphone apps that quietly track users’ locations. While fitness-tracking devices themselves don’t collect that type of data, their apps often do.

But advocates for personal tracking argue that this type of information is ultimately empowering, both for the individual and the population at large. As Nick Felton, the tracking maestro who logged his every conversation, told The Post last week:

I have found working with data to be a great shorthand for collecting experiences, and aggregation a fascinating technique for gaining new perspectives that are unattainable when face to face with our daily routines.

That’s just one guy. Imagine what the data of a million guys could tell us.

Caitlin Dewey runs The Intersect blog, writing about digital and Internet culture. Before joining the Post, she was an associate online editor at Kiplinger’s Personal Finance.

Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.

To pause and restart automatic updates, click "Live" or "Paused". If paused, you'll be notified of the number of additional comments that have come in.

Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.